Furnace control system with dual speed blower



May 20, 1958 2,835,447

FURNACE CONTROL SYSTEM WITH DUAL SPEED BLOWER w. H. CALAHAN Filed Nov. 6, 1953 A mwmM a flu W J I m Mg A A United States Patent 6 FURNACE CGNTRDL SYSTEM WITH DUAL SPEED BLOWER William H. Calahan, Fort Thomas, Ky., assignor to The Williamson Company, a corporation of Blue Application November 6, 1953, Serial No. 390,505

2 Claims. (Cl. 236--9) The present invention relates to forced air heating apparatus, and is particularly directed to a system for automatically controlling the operation of a furnace burner and an associated blower so that the heating apparatus is effective to maintain a uniform temperature within the space being heated.

At the present time many furnaces are provided with apparatus for automatically supplying a fuel such as oil, gas or coal to a furnace whenever the room temperature drops below a predetermined level and for stopping the input of fuel when the room temperature again reaches the predetermined level. One of the principal differences inherent in these systems is that the temperature is not kept uniform throughout the space being heated. This is especially true during warm weather when conventional heating apparatus supplies a short blast of very hot air to the room, opening the room thermostat before the air in many parts of the room has become heated. With previous heating systems these cool areas must be tolerated or else the temperature of other parts of the room must be raised to an uncomfortable level.

A further disadvantage of previous heating control systems is that there is a tendency for the temperature in the room to overshoot the desired level during each operating cycle of the furnace. The wide temperature fluctuations which thus result not only cause substantial discomfort to the occupants of the room, but also cause very inelficient operation of the heating system. It will readily be appreciated that while a certain amount of heat is dissipated from a room kept at a predetermined temperature, 72 for example, a very much larger amount of heat is lost when the room temperature overruns the desired level and rises to 80 for instance.

The principal object of the present invention is to provide a control system for a furnace and blower unit which is effective to regulate the operation of the furnace and blower so that the temperature within the space being heated is maintained at all times at substantially the desired level. Consequently, the air within the room never becomes overheated to cause either discomfort to the occupants or excessive heat loss to the outside air.

The present invention is predicated upon the concept of providing a control system for a forced air furnace which in mild weather terminates the burner operating cycle whenever the temperature of the heated air reaches a moderate preselected, or low limit, value, for example 150. The burner operating cycle is thus terminated, even though the heat requirements of the room have not been filled and the room has not reached the desired temperature. The blower, during mild weather, is operated at its lower speed and supplies a moderate volume of warm air to the room. In the present system, the blower continues to operate for a period after the burner has been shut off, more particularly, until the air temperature in the furnace drops to a relatively low level, such as 110.

1 have found that often duringthis period, when only the blower is operating, sufficient heat is supplied to the room to satisfy its requirements; Whereas, if the burner were operated until the room heat requirements were satisfied, the blower, during its period of operation, would then deliver a substantial surplus of heat to the room, causing the room temperature to overrun its de= sired value by an appreciable amount. In cold weather, however, the burner operation is terminated only when the room temperature is satisfied, or the air temperature exceeds a high safety limit, and the blower is operated at its higher speed to supply a large volume of very warm air to the room. During cold weather the heat losses of the room are sufficiently large so that the heat supplied by the blower after the burner ceases operation is not greatly in excess of the heat loss of the room, and consequently, no large temperature overrun is caused.

More specifically, a heating system constructed in accordance with the present invention comprises a furnace including a burner, such as an oil burner or a gas burner, and a blower operable at either of two speeds. The blower preferably draws cold air from the room and forces it through the plenum chamber of the furnace where it is heated and from which it passes into the rooms of the house. A control circuit is provided for regulating the operation of the burner and blower, or more precisely, the blowerand the supply of fuel to the burner.

The control circuit includes two thermostats, an outdoor thermostat and a room thermostat. The outdoor thermostat is adjusted to close at a selected. outdoor temperature, for example 20. This particular temperature, as will be explained below, is the temperature at which the manner of operation of the heating system is changed from one particularly adapted for mild weather to one particularly adapted for very cold weather. In any installation the optimum temperature at which this change should be made depends upon may factors, such as the size of the house being heated, the size of the furnace and the capacity of the blower. The room thermostat is readily adjustable and can be set to close whenever the temperature in the room drops below a comfortable level, such as 72. The room thermostat functions to close a circuit for operating the burner and consequently the blower. The outdoor thermostat conditions the control circuit so that in mild weather the operation of the burner, which is initiated by the closure of the room thermostat, is terminated whenever the air in the plenum chamber reaches a moderate value, while in extreme weather the operation of the blower is terminated only when the room temperature reaches the selected level, or the air in the furnace exceeds a maximum safe value. The outdoor thermostat also conditions the blower circuit so that the blower is operated at low speed in mild weather and at high speed in cold weather.

In mild weather, if the room temperature is not satisfied when the burner operating cycle is terminated by the temperature rise within the plenum chamber, the blower will continue to operate, transfering heat stored in thefurnace to the room. If the room temperature is not raised sufiiciently by this additional heat, the fuel supply means is re-energized when the plenum chamber temperature drops to a value somewhat below the moderate, or low limit, value at which the fuel supply means was de-energized.

One of the principal advantages of the present system is that temperature overruns are substantially eliminated, especially during mild weather when they are normally most noticeable. The elimination of appreciable overruns is largely due to the fact that in mild weather the plenum temperature is kept'at relatively low values. Consequently, only a small amount of heat is stored in the furnace walls during the periods of burner operation, and there is no accumulation of a tremendous quantity thedesired temperature, it often has alreadyproduced allofthe-required heat and when the heat stored in the furnace is supplied to the-room it is effective to raise the room temperature the final amount needed to bring it to the desired level. If, on the other hand,- the burner had-operated until the-room was at the desired temperature, this stored heat could only cause an appreciable temper-at-ureoverrun;

A still furtheradvantage ofthe present invention is that it provides-extremely eflicient operation-of the heatingsystem; A maximumamount of the total heat generated -by-the burner-is utilized. For-not only'isthe heat produced by the burner-during its operation transferred to--theroom;-but at the end of eachhurner operating cyclea maximum amount of the heat stored'in the-furnace is utilized'to heat the room. Moreover, in the present heating-system-the delivery of this-heat to the room does not result in an appreciable temperature overrun, so that the heatis not wasted-through accelerated heat loss from the room.

Another advantage of the present-heating system is thatin moderate, as well as incold weather, the temperature within all portions of the space-being heated remains uniform. That is, during moderate-weather a small volumeof airis supplied totheroom so that-the air has time-to circulate thoroughly throughout the room, heating all portions of it at the same-rate at which-the thermostat is heated. The room-is thus free from-any cold areas, since by the time-the room'thermostat has closed, the entire room hasbeen evenly heated to the desired temperature:

A still further advantage of the present invention is that the-control system comprises-a minimum number of control elements and hence is both economical to install and is extremely'reliable in operation.

These andotheradvantagesof the present invention will be apparent-from a consideration of the following detailed 'deseriptionof the drawings illustrating a preferred embodiment of the invention.

In the drawings Fig-urel is a diagrammatic view of a heating system constructed in accordance with the present invention, together -with-a--superimposed'schematic circuit diagram ofthe control apparatus.-

Asshown in'Figure l, a heating system constructed in accordance withthepresent-invention includes a furnace having combustion chamber 11 disposed interiorly of a warm air duct or bonnet12. Warm air is supplied to bonnet12'by a blower 13 which draws air through a cold air-return-air duct 14-having a register 15' opening into a room. The airdischarged into the bonnet from blower 13 is heated and forced through warm'air duct 16-to register 17 .disposed .within the rooms being heated.

Combustion chamber 11 houses a burner 18 which may be of any conventional construction adapted to burn a fuel such as gas or oil. In the presentembodiment burner 18 is an oil burner supplied with fuel oil by means of pipe 20 interconnecting, the burner with a pump 21. The pump is effective to draw oil from a reservoir (not shown) through pipe 22 and supply it to-the burner 18 under the proper pressure. Pump 21 is driven by means ofan electric motor 23' mechanically connected to the pump throughany suitable coupling 24.

The control apparatus for the furnace includes a room thermostat 25 located within the room being heated and an outdoor thermostat 26 located exteriorly of the building,preferably-along a north wall-thereof. These thermcstats can be of any suitable;type comprising for example a bimetallic strip 27 having a stationary portion 28 and a movable contact arm 30 which shifts toward and away from two stationary contacts in accordance with tempera ture changes of the surrounding air. The contact arm 30 of the room thermostat is adapted to complete a circuit across contacts 31 and 32 when the temperature drops below a selected value, for example 72". Similarly, outdoor thermostat Z6 is adapted to close a circuit between contacts 33 and 34 whenever the outdoor temperature drops below a predetermined value, for example 20.

Voltage for the control system is taken from house main lines 35 and 36, line 36 preferably being the grounded connection. Primary windings 37 and 38 of step down transformers 4 and 4.1. are connected across these lines. Line 36 is also connected through lead 42 to terminal 43 of motor 23 and through lead 44 to common terminal 45 of blower 13. Line 35"is joined through leads 46 and 47 to terminal 48' of high limit switch 50 and through lead 51 to terminal 52 of combined low limit and blower switch 53.

High limit switch 50' and low limit-blower switch 53 are thermostatic switches including a temperature sensitive element in communication with the plenum chamber of the furnace. These switches may be of any suitable construction including, for example, stationary contacts and a movable bimetallic element. However, in the embodiment shown, high limit switch 50 and low limit-blower switch 53 include mercury filled bulbs 54, 55, mounted on a frame 56 connected to a helical thermostatic strip 57. As the temperature within the plenum chamber rises, helical strip 57 causes mercurybulbs 54 and to tilt, closing the switches in a manner to be explained later.

The secondary winding 58 of transformer 41 is connected through lead 61 to relay coil 'll-which is in turn joined to contact 31'of'the room thermostat 25 through lead 62. The other contact 32 of the room thermostat is returned to primary 58 through lead.63; The energization of relay coil 69 controls the movement of armature 64 which is adapted to bridge contacts 65 and 66 when the relay coil is energized by the closure of the room thermostat. Contact 65 is joined to terminal 67 of motor 23 through line 68, while contact 66 is connected through line 70 to contacts 71 and 72 of conditioning relay 73. Relay 73 comprises armature 74 carrying four insulated contact arms 75, 76, 77, and 78. Control coil 80 of the relay is connected to secondary 81 of transformer 40 through lead 82 and to contact 33 of the outdoor thermostat 26 through lead 84. Contact 34 of the room thermostat is returned to the opposite side of secondary winding 81 through connector 85. Conditioning relay 73 is constructed so that when coil 80 is de-energized, arms and 76 respectively bridge contacts 71 and 86, 87 and 88; and when coil is energized, arms 77 and 73 respectively bridge contacts 72 and 90, and 91 and 92.

The function of relay 73, as will be explained in greater detail below, is to condition the control apparatus so that when the outdoor temperature is mild, that is, above a predetermined value, such as 20, the low limit switch de-energizes the pump motor 23, stopping the operation of burner 18 whenever the air temperature in the plenum chamber reaches a predetermined value. Relay 73 also functions to condition the blower energization circuit so that the blower operates at low speed whenever the air in the plenum chamber is warm enough to close the blower switch. On the other hand, when the outdoor temperature is below 20, relay 73 conditions the circuit so that the blower 13is operated at a high rate whenever the blower switch is closed, and additionally, the low limit switch is removed from the pump motor circuit so that for practical purposesthe burneris operated until the room thermostat is satisfied, or alternatively until the air temperature in the plenum exceeds the setting of the '5 high limit safety switch, which setting is generally in the neighborhood of 250.

The circuit arrangement by which relay 73 is efiective to cause the described operation includes lead 94 interconnecting contact 87 and low terminal 95 of the blower. High terminal 96 of the blower is joined to contact 91 through connector 97. Contacts 88 and 92 are tied together by conductor 98 and are joined to'contact 100 of the low limit-blower switch through lead 101. The third terminal 103 of the low limit-fan switch is joined to contact 86 by connector 104; and terminal 105 of the high limit switch is connected to contact 90 through lead 106.

The low limit portion of switch 53, including contacts 103 and 52, is normally closed and should be set to open when the air temperature within the plenum chamber reaches a value at which it is efiective to convey a moderate amount of heat into the room. This value is not absolutely critical, but preferably is generally of the order of 50 percent of the maximum air temperature which the furnace can produce. It have found that a temperature setting of approximately 150, provides very effective operation. Once the low limit switch has opened it preferably does not close again until the plenum temperature drops approximately 15 below the temperature at which the switch is set to open; in the present embodiment this temperature is 135. The blower controlling portion of switch 53 including contacts 103 and 52 should be set to close at a plenum chamber temperature some what less than the low limit temperature, for example 110. That switch should also be adjusted so that it opens when the temperature in the plenum chamber drops to a still lower value, for example 90, following operation of the burner. The high limit safety switch is pref erably adjusted so that it opens at around 250".

When both the room thermostat and outdoor thermostat are open, it can be seen that neither the pump nor blower is energized. The motor circuit is open at relay armature 6d since coil 60 of that relay is de-energized because of the condition of room thermostat 25. Similarly, since the air temperature in the bonnet 12 is below 110, the blower circuit is open at blower switch 53. However, when the room thermostat is closed, the circuit to relay coil 60 is completed and armature 64 bridges contacts 65 and 66. This completes the circuit to motor 23 through leads 42, 68, 70, relay arm 75 bridging contacts 71 and 86, lead 104, low limit switch 53, and leads 51 and 46.

When the air within the plenum chamber is heated, to 110 due to the operation of burner 18, a circuit is completed through contacts 10352 of fan switch 53, low limit contacts 160-52 remaining closed. The closure of the fan switch completes the circuit to the low speed terminal 95' of the blower 13. It will be understood that blower 13 is provided with a two speed motor which operates at one speed when the circuit to terminal 95 is completed and is operated at an appreciably higher speed when the circuit to terminal 96 is completed. The blower is thus effective to supply either a moderate or a large volume of heated air to register 17, depending upon which of the mot-or terminals is energized. When the blower switch is closed the circuit to terminal 95 is completed through lead 94, relay arm-bridging contacts 87 and 88, lead 98, blower switch 53 and conductor 51. The other blower terminal 45 is connected to ground power line 36 at all times.

When the outdoor thermostat is closed, energizing coil 80 of circuit conditioning relay coil 73, armature 74 is urged downwardly so that arms 77 and 78 engage their respective sets of contacts. With, the relay armature in this position and the room thermostat closed, the circuit to motor 23 is completed through lead 68, relay arm 64, line 70, relay arm 77 bridging contacts 72 and 90, connector 106, high limit switch 50 and leads 47 and 46.

When the burner 18 has heated the air in plenum pharnber 11 to 110, the blower switch closes, completing the circuit to the high speed blower terminal 96, it being noted that the circuit to the low speed blower terminal remains open at contacts 87 and 88 of the conditioning relay. The circuit to high speed terminal 96 includes lead 97, relay arm '78, line 101, blower switch 53, and leads 51 and 46. With the outdoor thermostat closed, the burner continues to operate until the room thermostat opens or until the air within the plenum chamber reaches the high limit setting, preferably of the order of 250. At this time high limit switch 50 is actuated, opening the circuit to pump motor 23. However, the blower continues to operate at high speed until the temperature in the plenum chamber drops below at which time the blower circuit will be opened at blower switch 53.

in operation, so long as the room temperature is above the desired level, for example 72, neither the blower nor the pump motor is operated, the circuit to the blower being open at fan switch 53 and the circuit to the pump motor being open at relay armature 64. Assuming that it is a relatively mild day with the outdoor temperature above the 20 setting of thermostat 26, the outdoor thermostat is open, relay coil 80 is de-energized, and the armature 74 of circuit conditioning relay 73 is in its upper position.

When the temperature within the room drops below the room thermostat setting of 72, room thermostat 25 closes, completing the circuit to relay coil 60, thus forcing armature 64 into engagement with contacts 65 and 66. This completes the circuit to pump motor 23 and burner 18 is thereby provided with fuel. Operation of burner 18 causes the temperature of the air within the plenum chamber to rise and when this air reaches a temperature of corresponding to the setting of blower switch 53, that switch is tripped, completing the circuit through relay 73 to the low speed terminal of the blower. The blower thus begins to supply a moderate quantity of warm air to the room. Burner 18 will continue to operate until the occurrence of either one of two events. In the first place, if the heat requirements of the room are not met, that is the room temperature isnot raised above 72, continued operation of the burner causes the temperature in the plenum chamber to rise until it reaches the low limit setting of 150. At this temperature, the low limit switch is actuated, opening the circuit to motor 23. This terminates the operation of burner 18. However, during its period of operation the burner has been efiective to generate a quantity of heat now stored in chamber 11 and in the furnace walls. This heat is effectively supplied to the room by the continued operation of the blower, which is terminated only when the temperature in the plenum chamber drops below 90, opening the blower switch.

However, if the room temperature is not raised to the desired level by the time the plenum chamber temperature drops below low limit switch 53 is again closed to restore the circuit to the pump motor and start burner 18 in operation again. The operation of the burner will continue until either the air temperature in the plenum rises above or alternatively, until the heat requirements of the room are satisfied and thermostat 25 opens to break the motor circuit at relay armature 64. It has been found that frequently the room thermostat, which is still closed when the burner operation is stopped by switch 53, will be opened by the time the plenum temperature drops to 135 indicating that a sufiicient amount of heat has been supplied to the room by the blower. In this case the burner is not re-energized when the plenum temperature drops, but nevertheless a small amount of heat is still supplied to the room by the continued blower operation. In mild weather this small amount or" heat only slightly exceeds the rate of heat loss from the room so that no excessive temperature fluctuation is caused, although a maximum amount of the heat produced by the burner during its period of operation is utilized.

If, instead of being mild, the outdoor temperature is below thermostat 26 is closed, energizing coil 80 of the conditioning relay. When the room thermostat closes, pump motor 23 is energized, and the high speed terminal of the blower motor is connected in series with blower switch 53. The circuit to this terminal is completed when burner 18 raises the air temperature in the plenum chamber to 110, at which time the blower switch is closed, and the blower supplies a high volume of air to the room. Under these conditions, the burner continues to operate until either the room temperature rises above 72, opening thermostat 25 and relay armature 64, or the plenum chamber exceeds the high limit safety setting of 250. However, even after the operation of the burner is terminated, the fan will continue to operate, delivering a high volume of heated air to the room until the temperature in the plenum chamber drops below 90. Since during colder weather the heat losses from the room are large, even this increased volume of air supplied to the room after it has reached the desired temperature, does not convey heat to the room at a rate greatly in excess of the heat loss. Consequently, undue temperature fluctuations in the room are avoided in cold weather as well as in mild weather; yet substantially all of the heat generated during the operation of the burner is utilized.

From the foregoing description of the general principles of my invention and the detailed disclosure of a preferred embodiment, those skilled in the art will readily comprehend the various modifications to which my invention is susceptible. For example, while I have shown but a single outdoor thermostat controlling a single conditioning relay to place a low limit switch in the fuel pump energization circuit, several thermostats, conditioning relays, and limit switches could be employed. That is, a plurality of outdoor thermostats, each set at a different temperature, could be provided, each thermostat governing the operation of a conditioning relay effective to insert a limit switch having a unique temperature setting in circuit with the burner fuel supply pump. Consequently, burner operation would be terminated at one of a plurality of different plenum temperatures in accordance with the outdoor temperature. If desired, a multispeed fan could be used in conjunction with this arrangement so that various fan speeds could be employed to match 'the various plenum temperatures.

Furthermore, while in the embodiment shown the burner operation is regulated by control of a fuel pump motor, it will be appreciated that other electrically actuated control means could be used. For example, if a gaseous fuel is to be burned, leads 42 and 68 could be connected to an electrically responsive valve rather than to pump motor 23.

Having described my invention, I claim:

1. A control system for a furnace including a burner, electrically responsive means for supplying fuel to the burner and a blower effective to force air through the furnace into a room, said blower being selectively operable at either a low speed or a high speed, said control system including a room thermostat in circuit controlling I said fuel supply means when the outdoor temperature drops below a predetermined level, blower switch means in circuit controlling relationship with said blower, said blower switch means being adapted to close at a temperature lower than the temperature at which said low limit switch recloses, means responsive to the condition of said outdoor thermostat in circuit connection with said blower for conditioning said blower for operation at either a high speed or a low speed in response to the closing of said blower switch means.

2. A control system for a furnace having a burner, electrically responsive means for supplying fuel to the burner, and a blower effective to force air through said furnace and into a room, said blower being selectively operable at either a high speed or a low speed, said control system comprising a room thermostat and an outdoor thermostat, each of said thermostats having contacts respectively adapted to close when the room and outdoor temperatures drop below predetermined levels, said room thermostat being effective upon closure to energize the fuel supply means for operation of the burner, first switch means responsive to the furnace temperature for energizing said blower, low limit switch means operable at a predetermined furnace temperature higher than that at which said first switch means are actuated, high limit switch means operable at a substantially higher furnace temperature than the low level switch means, the outdoor thermostat being effective when open to place said low limit switch means in the energizing circuit of the fuel supply means whereby said fuel supply means are de-energized by the low limit switch means whenever the air in the furnace reaches said predetermined temperature, the outdoor thermostat being effective when closed to remove said low limit switch means from circuit controlling relationship with said fuel supply means and to insert the high limit switch means in the energization' circuit of said fuel supply means, said outdoor thermostat further being elfective when open to condition said control system whereby said blower is operated at a low speed in response to the actuation of said first temperature responsive means, said outdoor thermostat being effective when closed to condition said control system whereby said blower is operated at a high speed in response to the closing of said first switch means.

References Cited in the file of this patent UNITED STATES PATENTS 1,883,015 Shipley Oct. 18, 1932 2,402,177 Miller June 18, 1946 2,425,998 Crise Aug. 19, 1947 2,615,634 Harbin Oct. 28, 1952 

